This invention relates to a safety device in a circular knitting machine, in particular a high speed single cylinder stocking knitting machine. More specifically, the invention relates to a safety, or protection, device adapted to prevent breakage of the butts of knitting instruments, particularly the butts of the intermediate jacks in a single cylinder stocking knitting machine.
It has been found that as the rotational speed of a single cylinder circular stocking knitting machine increases, a behavior is evidenced with increasing frequency which does not occur at normal operating speeds, namely the breakage of the butts of the intermediate jacks, which are located, as is well known in the art, between the needles and respective pattern jacks thereunder.
It has also been found that the occurrence of such breakage is more frequent, the higher is the speed at which a given machine is caused to rotate.
This behavior, which is not easily explained, represents a considerable difficulty encountered in designing and building high speed machines, in accordance with a modern trend.
Attempts have been made to obviate this problem by providing intermediate jacks of a special type, i.e. of a tougher material. During tests carried out with intermediate jacks made of such a material, an actual reduction in the number of butt breakages has been observed, although the problem failed to be eliminated altogether. On the contrary, it became evident that, while at a given high speed the number of breakages of the special type butts was statistically lower than in the case of butts made of the usual material, still the problem tends to reappear substantially as before at higher speeds.
On the other hand, it will be appreciated that when even a single butt of an intermediate jack breaks, it is no longer possible to carry on knitting, and the machine must be stopped, the product discarded, and the broken intermediate jack must be replaced, with an obvious loss of output.
As regards the breakage itself, this invariably occurs for a certain length along a direction parallel to that of the butt at the area of the intermediate jack where the stem of the intermediate jack joins the butt portion. This condition was also investigated, but no satisfactory cure had been found heretofore to prevent breakage in high speed machines, i.e. speeds in excess of 400 RPM.